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From: email@example.com (Tom Bruhns)
Subject: Re: Heart Rate Sensor Circuit - Circuit Explanation
Date: 23 Oct 2002 00:02:06 -0700
NNTP-Posting-Date: 23 Oct 2002 07:02:06 GMT
firstname.lastname@example.org (Padraig Fitzgerald) wrote in message news:...
> I have found this circuit on the net but I'm looking for information
> on it's operation.
> It's located at http://www.elecdesign.com/1997/dec1597/ifd/1IFD.pdf .
> Basically it's the Lowpass filter part of the circuit surrounding
> OpAmp A1 and BJTs Q4 and Q5 that I am confused about. I have searched
> a lot of electronic books and Filtering techniques but have never seen
> anything like this. I was wondering if anyone out there could shed
> light on its operation, perhaps equations/formulae etc or at least
> point me in the direction of some literature which would explain this.
Note that both Q4 and Q5 have their base and emitter tied together.
That means they will simply behave as the base-collector junction,
which generally is a low-leakage diode. In series circuits of diodes
and resistors (R2), it's difficult to write an analytic expression for
the current versus voltage, but it's often enough to know that the
diode looks like a very high resistance for low voltage. In other
words, the R2 plus diodes is essentially not there for low voltages.
I presume the idea is to bias things: they are the only path for the
input bias current for the op amp inputs they are connected to on the
left. They also provide some feedback for larger signals.
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